X-ray reflectivity from curved surfaces as illustrated by a graphene layer on molten copper

Abstract

The X-ray reflectivity technique can provide out-of-plane electron-density profiles of surfaces, interfaces, and thin films, with atomic resolution accuracy. While current methodologies require high surface flatness, this becomes challenging for naturally curved surfaces, particularly for liquid metals, due to the very high surface tension. Here, the development of X-ray reflectivity measurements with beam sizes of a few tens of micrometres on highly curved liquid surfaces using a synchrotron diffractometer equipped with a double crystal beam deflector is presented. The proposed and developed method, which uses a standard reflectivity θ–2θ scan, is successfully applied to study in situ the bare surface of molten copper and molten copper covered by a graphene layer grown in situ by chemical vapor deposition. It was found that the roughness of the bare liquid surface of copper at 1400 K is 1.25 ± 0.10 Å, while the graphene layer is separated from the liquid surface by a distance of 1.55 ± 0.08 Å and has a roughness of 1.26 ± 0.09 Å.

Graphene at Liquid Copper Catalysts: Atomic-Scale Agreement of Experimental and First-Principles Adsorption Height

“…The advantage of using this Cu inflection point is a "gap" distance that does not dependent on the Cu roughness. Minor deviations of the below-quoted value compared to previous reports [7,17] can be explained by the much shorter qz range available for fitting (between 0.4 -1.6 Å -1 ) in these preceding studies. Machine-Learning Potential.…”

“…The XRR data from the curved liquid Cu surface were treated according to the method described recently in ref. [17] to eliminate the curvature's effect and extract the normalized reflectivity profiles. The resulting XRR curves were fitted using Refl1D software to a slab model consisting of one (in case of bare Cu) or three (in case of Cu covered with Gr) layers: a Cu substrate, a void, and a C monolayer.…”

“…The XRR scans were treated according to the method described recently in ref. [17], which not only accounts for the effect of the liquid surface curvature but also takes advantage of it. This, together with an improved reactor setup (e.g., larger sample surface) and measurement methodology, allowed to extract the normalized reflectivity profiles up to an unprecedented maximum value of the momentum transfer perpendicular to the surface, qz.…”

“…With all details provided in the SI and Ref. [17], this fit yields the lateral average electron density profile against the surface depicted in Figure 2b. From this profile, the "gap" distance is determined as the distance between the inflection point of the Cu density and the center of the Gr slab/layer, as indicated by the dashed lines in Figure 2b.…”

“…Improved operando measurement setups and subsequent data analyses provide more and increasingly accurate atomic-scale observables. Here, we illustrate this by now achievable widerange q-vector in situ XRR measurements in a tailored reactor setup [7,11,17] that provide the adsorption height of a Gr monolayer above the liquid Cu catalyst (the so-called "gap") at unprecedented accuracy. On the modeling side, the advent of machine learning (ML) potentials constitutes a breakthrough [18,19,20].…”